"Asteroid immigration from other star systems occurs because the Sun initially formed in a tightly-packed star cluster, where every star had its own system of planets and asteroids," says Dr. Helena Morais, an astronomer with São Paulo State University.

"How the asteroid came to move in this way while sharing Jupiter's orbit has until now been a mystery," explains Fathi Namouni, with Observatoire de la Côte d'Azur, and lead author of the study. “It would be very interesting to go and observe it more and understand its composition.”

“Before, we only had to work to explain solar system phenomena with the objects that are in the solar system and thought to be part of the solar system all the time,” he said. “Now we have new sources of material that actually influenced the solar system – and so the solar system did not grow up in isolation.”

The new study has discovered the first known permanent immigrant to our Solar System. The asteroid, currently nestling in Jupiter's orbit, is the first known asteroid to have been captured from another star system. The object known as 'Oumuamua was the last interstellar interloper to hit the headlines in 2017. However it was just a tourist passing through, whereas this former exo-asteroid—given the catchy name (514107) 2015 BZ509—is a long-term resident. The permanent visitor is about 3km across and was first spotted in late 2014 by the Pan-Starrs project at the Haleakala Observatory in Hawaii.

All of the planets in our Solar System, and the vast majority of other objects as well, travel around the Sun in the same direction. However 2015 BZ509 is different—it moves in the opposite direction in what is known as a 'retrograde' orbit.

"If 2015 BZ509 were a native of our system, it should have had the same original direction as all of the other planets and asteroids, inherited from the cloud of gas and dust that formed them," Namouni said.

However the team ran simulations to trace the location of 2015 BZ509 right back to the birth of our Solar System, 4.5 billion years ago when the era of planet formation ended. These show that 2015 BZ509 has always moved in this way, and so could not have been there originally and must have been captured from another system.

Stellar nursery NGC 604 (NASA/HST), where star systems are closely packed and asteroid exchange is thought to be possible. Asteroid (514107) 2015 BZ 509 emigrated from its parent star and settled around the Sun in a similar environment. Credit: NASA / Hubble Heritage Team (AURA/STScI)

"The close proximity of the stars, aided by the gravitational forces of the planets, help these systems attract, remove and capture asteroids from one another."

The discovery of the first permanent asteroid immigrant in the Solar System has important implications for the open problems of planet formation, solar system evolution, and possibly the origin of life itself.

Understanding exactly when and how 2015 BZ509 settled in the Solar System provides clues about the Sun's original star nursery, and about the potential enrichment of our early environment with components necessary for the appearance of life on Earth.